1. Field of the Invention
The present invention relates to an image heating apparatus for using as an image heat fixing apparatus in an image forming apparatus such as a copying apparatus, a laser beam printer, a facsimile apparatus and the like.
2. Related Background Art
As a fixing apparatus to be incorporated in an image forming apparatus, already commercialized is an apparatus including a ceramic heater, a heat resistant sleeve rotating in contact with the ceramic heater, and a pressure roller constituting a fixing nip portion with the ceramic heater through the heat resistant sleeve. A recording sheet bearing an unfixed toner image is pinched between and conveyed by the heat resistant sleeve and the pressure roller, whereby the unfixed toner image is heat fixed to the recording sheet. Such fixing apparatus, having a low heat capacity, provides advantages of requiring a short time to reaching a fixable temperature and a low electric power consumption during a stand-by state awaiting an instruction for printing.
For improving the fixing property in an image heating apparatus of the above-described film heating type, as disclosed in FIG. 11 of Japanese Patent Application Laid-open No. 2001-27858, a configuration of forming a pressure distribution in the fixing nip portion so as to be larger in a downstream side than in an upstream side in the sheet conveying direction.
It is however found that, even when the pressure at the downstream side is made larger in the pressure distribution of the nip portion, an unnecessarily high pressure is applied while the toner is insufficiently fused in case heat transmission is not well balanced. Stated differently, it is found necessary, in order to secure satisfactory fixing property, to optimize the pressure distribution and the temperature distribution within the fixing nip portion, in the sheet conveying direction.
Also in order to reduce the cost of the apparatus, it is desirable to achieve the optimization of the pressure distribution and the temperature distribution within the fixing nip portion, in the sheet conveying direction, by a simple structure.
It is also desirable to realize an image heating apparatus capable of suppressing the electric power consumption while securing the fixing property, by a simple structure.